A plant for continuous hot-dip galvanization of steel strip consists, inter alia, of a continuous furnace, a zinc bath (molten bath), an apparatus for adjusting the zinc coating thickness and a downstream cooling device. The steel strip is continuously annealed in the continuous furnace. The desired mechanical properties of the basic material are adjusted here by recrystallization of the steel. In addition, iron oxides FORMED in a preheating zone are reduced here. In a cooling zone downstream of the continuous annealing furnace, the strip is cooled in protective furnace gas (HNX) to a temperature close to the molten bath temperature. The protective furnace gas is intended to prevent the annealed strip from oxidizing prior to galvanization, which would considerably impair the adhesion of the zinc coating. The connecting piece containing protective furnace gas between annealing furnace and zinc bath is called furnace pipe.
In a conventional furnace pipe of a continuous strip galvanization plant, there are customarily deposits of zinc dust which, in particular in the event of vibrations occurring in the plant, drops in relatively large pieces onto the zinc bath and/or the steel strip and therefore causes surface defects (galvanization defects). It has been detected that the steel strip moving in the pipe in the direction of the zinc bath entrains protective furnace gas downwards, wherein the entrained protective furnace gas on the zinc bath surface absorbs zinc vapour which, as the entrained protective furnace gas rises, condenses or resublimates on the colder inner walls of the pipe and is deposited there as dust.
JP 7157853 (A) discloses an apparatus for removing zinc vapour in a pipe of a continuous strip galvanization plant. In order to remove the zinc vapour arising on the zinc bath surface, the furnace pipe is provided with injection openings (recirculating openings) and extraction openings arranged vertically therebelow. In a first exemplary embodiment, an individual injection opening and, vertically therebelow, an individual extraction opening are arranged in the pipe wall facing the upper side of the steel strip. Accordingly, an individual injection opening and, vertically therebelow, an individual extraction opening are likewise arranged in the pipe wall facing the lower side of the steel strip. In a second exemplary embodiment, an individual injection opening is arranged in a side wall of the pipe, while two extraction openings are provided vertically below said injection opening, the extraction openings being configured as longitudinal slots in conduits which penetrate the side wall of the pipe and extend over the entire steel strip width on the upper side and lower side of the steel strip.
With the apparatus known from JP 7157853 (A), a relatively large quantity of zinc vapour or zinc dust has to be removed from the extracted protective furnace gas. This is because, on the basis of the configuration and arrangement of the injection openings and extraction openings, it can be assumed that said known apparatus promotes the absorption of zinc vapour by the protective furnace gas entrained by the steel strip and promotes the dissemination of zinc vapour in the furnace pipe.